The present invention relates to a grease having improved radiation resistance. More particularly, the present invention relates to a radiation-resistant grease which has bentonite (thickening agent) and an alcohol, ketone or ester (gelling aid) incorporated in a base oil which is made of either m-(m-phenoxyphenoxy)diphenyl, pentaphenyl ether, a monoalkyltriphenyl ether or a monoalkyltetraphenyl ether.
Grease is employed in various machines at nuclear facilities such as power plants and fuel reprocessing plants, or at irradiation facilities such as cobalt-irradiating equipment and nuclear accelerators, and is subjected to two types of deterioration, one resulting from aging and the other caused by radiation exposure. The deteriorated grease is replaced at the time of periodic inspections and at other suitable occasions but the frequency of grease replacement is desirably held to a minimum in order to improve the degree of capacity utilization of the facilities and to reduce the frequency of exposure to radiation on the part of person. A situation therefore exists to require the development of a grease having superior resistance to radiation.
The greases currently employed in machines at nuclear facilities are roughly divided into two types: the silica gel grease that uses a petroleum based lubricant as a base oil, and the soap base grease that incorporates a petroleum based lubricant or a synthetic oil as a base oil. However, these conventional greases are not possessed of adequate resistance to radiation. If the silica gel grease using a petroleum based lubricant as a base oil is employed in machines at a high dose-rate irradiation site, the base oil solidifies as a result of radiation-induced polymerization and its lubricating property is lost within a short time to cause premature wear of the machine. On the other hand, the soap base grease which uses a petroleum based lubricant or a synthetic oil as a base oil will be softened as a result of exposure to an increased dose of radiation, and the fluidized grease will leak out of the machine. A possible cause of these phenomena is the loss of radiation resistance not only of the base oil and the thickening agent present in the grease but also of the gel structure which is formed as a result of interaction between the base oil and the thickening agent. It would therefore be clear that in order to provide a grease with enhanced radiation resistance, it is important that not only a base oil and a thickening agent having improved radiation resistance should be used but also the characteristic gel structure formed as a result of interaction between the base oil and the thickening agent should be stable to radiation.
The present invention has been accomplished in order to develop a highly radiation-resistant grease which is free from the defects of the conventional products.
The present inventors previously filed a patent application on a radiation-resistant grease that employed a mixed base oil having a low pour point and which performed satisfactorily at low temperatures (Japanese Patent Application No. 114657/1985). In this present invention, emphasis is placed on achieving better performance at high temperatures and its principal purpose is to develop a grease that employs a base oil made of a highly heat-resistant single component and which exhibits superior radiation resistance at elevated temperature.
The present inventors reported the superior radiation resistance of m-(m-phenoxyphenoxy)diphenyl and pentaphenyl ether oils in Journal of the Society of Atomic Energy of Japan, 26, 8, 718 (1984). In preparing a grease that is resistant to both heat and radiation, it is of course important to employ a base oil having high resistance to radiation but at the same time, it is indispensable to select a thickening agent that is not only heat- and radiation-resistant but also compatible with the base oil. Few reports have been made of greases using m-(m-phenoxyphenoxy)diphenyl as a base oil and there are no observations available that provide an immediate practical guide for choosing a thickening agent that is adapted for the preparation of a grease having high degrees of radiation- and heat-resistance.
The present inventors prepared three samples of grease that employed m-(m-phenoxyphenoxy)diphenyl as a base oil but which contained different thickening agents, i.e., urea, silica gel and bentonite. When these samples were irradiated with gamma-rays from .sup.60 Co for a total dose of 5,000 Mrad as room temperature, the greases using urea or silica gel as a thickening agent were liquefied but, on the other hand, the grease using bentonite remained almost intact and even after exposure to twice the dose level (10,000 Mrad), this grease experienced very small variations in worked penetration, oil separation, free acid content and dropping point. The greases employing urea or silica gel as a thickening agent were also liquefied when they were irradiated with gamma-rays for a total dose of 500 Mrad at 150.degree. C. but the grease using bentonite remained intact even in this case.
The present inventors found that equally good results were attained by using bentonite as a thickening agent when the base oil was made of pentaphenyl ether, a monoalkyltriphenyl ether or a monoalkyltetraphenyl ether rather than m-(m-phenoxyphenoxy)diphenyl. In addition, these greases exhibited the high levels of radiation resistance (.gtoreq.10,000 Mrad) that could not have been expected from the case of using the above-mentioned base oils alone, and this demonstrates the appreciable improvement in radiation resistance that was attained by using bentonite as a thickening agent. The reason for this effect is not completely clear but presumably the bentonite will form a special gel structure that is highly resistant to radiation.
The present invention has been accomplished based on the confirmation of the following findings: a grease having high radiation resistance can be attained from a composition that is basically the combination of a bentonite-based thickening agent and a base oil selected from among m-(m-phenoxyphenoxy)diphenyl, pentaphenyl ether, a monoalkyltriphenyl ether and a monoalkyltetraphenyl ether and which optionally contains at least one gelling aid selected from an alcohol, a ketone and a carbonate ester; this grease exhibits a superior performance when used in a mechanical snubber, a limit switch or any other devices that are to receive large doses of radiation or which are to be used in radioactive fields and at high temperatures.